Surgical procedures to correct myopia (nearsightedness), astigmatism and hyperopia (farsightedness) have been in widespread use for at least the past 20 years. Three surgical procedures to correct these problems are radial keratotomy, astigmatic keratotomy and hexagonal keratotomy.
Radial keratotomy is used to reduce or eliminate myopia which occurs when the cornea is too steeply curved or the eyeball is too long. Bluffed vision results because images focus in front of the retina. To correct for this, a series of radial incisions are made in the cornea peripherally around the central corneal zone of 3-4 mm in diameter. The incisions function to flatten the cornea and thereby move the focal point of the eye posteriorly toward the retina and ideally coincident with the retina. For optimum results, the incisions usually have to be at least 85-90% of the corneal thickness along the length thereof.
Astigmatic keratotomy corrects astigmatism caused by an irregularly shaped cornea. In an astigmatic eye, the shape of the corneal is more ovate than spherical, with the result being distorted vision. Surgical correction of the astigmatism is accomplished by placing a series of transverse incisions in opposite quadrants of the cornea. As with radial keratotomy, these incisions usually should be 85-90% of the corneal thickness.
Hexagonal keratotomy is used to correct hyperopia. Hyperopia occurs when the cornea is too flat or the eyeball is too short, whereby images focus behind the retina. To correct this problem, a "T-hexagonal" set of incisions are made in the cornea to form a six-sided geometric figure. The incisions cause the cornea inside the six-sided geometric figure to bow forward and thereby increase the curvature of the cornea and thereby move the focal point of the eye forward. This procedure has evolved since 1987 and, as currently modified by Dr. J. Charles Casebeer of Flagstaff, Ariz., has been quite successful.
At present, the above surgical procedures are for the most part done freehand by the surgeon in the following manner. The surgeon uses a pachometer, such as the Sonogage pachometer sold by Sonogage, Inc. of Cleveland, Oh., to measure the thickness of the cornea typically 1.5 mm temporal to the visual axis or several measurements are made around the periphery of the 3-4 mm central or optical zone. The lowest reading is used to set the depth of cut of a surgical knife to be used to make the incisions. The depth of cut typically is set to 85-110% of the measured minimum thickness of the cornea depending on the surgeon's previous experience.
The success of the above surgical procedures depends in part on the ability of the surgeon to guide the knife so that it is always perpendicular to the corneal surface. Cocking of the knife to either side will result in a shallower cut. Another problem is the increase in thickness of the cornea from approximately 500 microns at the optic center to approximately 580-600 microns at the periphery of the cornea. Accordingly, the percentage depth of the cut will usually decrease moving radially outwardly along the radial incision when performing radial keratotomy.